The Shaggin' Wagon is a type of match that gives people an opportunity to re-enter a speedrun tournament if they have been originally knocked out due to losing matches previously. This match type made its first appearance in I'm A Speedrunner 7 as one of the largest matches in terms of competitors in the I'm A Speedrunner series.
The concept of the Shaggin' Wagon match is similar to how a Double Elimination tournament works where a competitor can lose two matches before being eliminated. In other words, the Shaggin' Wagon acts as safety net for competitors during the knockout stage of the tournament.
The first Shaggin' Wagon match was originally planned to be a random selection of 18 competitors in IAS 7 to determine who would advance to Round 3 at a set deadline. However, due to competitors not being available at the time, only 7 out of the 18 people who were selected participated in the actual match.
The next major appearance of the Shaggin' Wagon was in the following tournament I'm A Speedrunner 8. This time, there were two smaller scale matches consisting of around three to four competitors composed of losers of Round 3. The second match came about due to the controversial debate involving the match between Ratchet5 and MuddyMaestro with the latter dropping out which resulted in the Shaggin' Wagon match with Nintendogen64 being the winner. While the match proceeded as planned, Ratchet5 dropped out as he was not happy about him getting back into the tournament.
This lead to Ratchet5's IAS? Nein! tournament which was composed with many, many Shaggin' Wagon matches. In fact, the core focus of tournament revolved around these matches so much that he had to explain across several videos how the tournament would operate with them in mind.